The invention pertains generally to the art of converting an analog signal into a digital form and is more specifically directed to a voltage control oscillator having ratiometric and temperature compensation.
The consequences of environmental regulations and the world's dwindling fuel supply have necessitated the precision control of air/fuel ratio for internal combustion engines. In response to this need there have been developed electronic fuel injection systems which accurately control the fuel metering requirements of an internal combustion engine based on the instantaneous operating parameters thereof. With recent advances in digital electronics in general and particularly with the advent of small fast microprocessor units prevalent forms of electronic control units will in the future be digital in operation.
Conventionally, microprocessors are adapted to receive digital inputs in the form of binary words or binary logic signals. Because of the inherently analog nature of the engine operating environment and its parameter sensors, an analog-to-digital converter is necessary for transforming this information to a microprocessor in a readable form.
A convenient way of accomplishing analog-to-digital conversion with a microprocessor utilizes a voltage controlled oscillator which generates a square wave whose frequency is representative of the magnitude of the analog input parameter. Advantageously, the switching levels of the square wave can be read directly by the microprocessor as a form of serial logic signal requiring only one input bit. By timing the duration between transitions in the square wave with a counter, the microprocessor can internally convert the elapsed time into a digital word which can be processed further.
One significant problem with the analog-to-digital conversion of the operating parameters of an internal combustion engine is the dependence on the analog signals with supply voltage variations. The supply or battery voltage in an internal combustion engine system is relatively unregulated because of changing speed and load conditions. Starting conditions and/or air conditioning loads can produce substantial changes in the voltage supply and hence the analog output from the sensors sampling the instantaneous operating parameters of the engine. Therefore it would be highly desirable for any conversion technique to be provided with compensation for variations in supply voltage.
Further the engine compartment of an internal combustion engine is a very hostile environment for electronics. Many components and electronic systems are temperature dependent and the range over which these components must operate is extensive. Therefore it would be highly desirable for any compensation technique to further include compensation for temperature variations.